The invention is related to the field of DC-DC converters, and in particular to DC-DC converters with pulse generators shared between PWM and PFM modes
DC-DC converters are commonly used to supply DC power to electronic devices, such as personal computers, hand-held devices, and the like, and are available in a variety of configurations for deriving a desired DC output voltage from a given source of DC input voltage. For example, a buck mode or step-down DC-DC converter is often used to supply a regulated DC output voltage, whose value is less than the value of the DC source voltage. A typical step-down DC-DC converter includes one or more power switches, current flow paths through which are coupled between a DC input voltage terminal and a reference voltage terminal (e.g., ground), and the common or phase node between which is connected through an output inductor to an output voltage node, to which a storage capacitor and the powered load/device are connected. By controllably switching the power switches on and off, the upstream end of the output inductor is alternately connected between the DC input voltage and the reference voltage. This produces an alternately ramped-up and ramped-down output current through the output inductor to the output node, and causes a stepped-down DC output voltage to be delivered to the load. The DC-DC converter may be configured as a voltage mode converter or a current mode converter.
In addition to the above-described voltage mode and current mode DC-DC converters, there is an additional type of DC-DC converter, known as a constant on-time or pulse-frequency modulated (PFM) DC-DC converter. This type of converter is typically used in applications where load current demand is relatively small, as in the case of a “sleep” or “quiescent” mode of operation of a notebook computer, for example. A PFM converter includes a control loop having a voltage comparator, the output of which is used to create a triggering signal for a one-shot that sets a constant on-time for a relatively narrow pulse-width switching signal upon which switching times of the power switches are based. Because of its relatively narrow pulse-width, the switching signal provides the PFM mode converter with ability to turn on the power switches for very short time intervals—just sufficient to meet the very low current demands of the load, thereby saving power and prolonging battery life. This mode of operation is customarily referred to as discontinuous conduction mode (DCM), because for each switching pulse, the current delivered through the inductor is allowed to reach zero, and the power switches are opened when this happens.